CVE-2021-23995Network attack vector

Arbitrary code execution via Responsive Design use-after-free

Published Jun 24, 2021 · Updated Aug 3, 2024

Use-after-free in Mozilla Firefox before 88, Firefox ESR before 78.10, and Thunderbird before 78.10 allows attackers to execute code. Responsive Design Mode retains and later dereferences references to objects after their storage has been freed, causing memory corruption. Exploitation requires a user to enable Responsive Design Mode and interact with attacker-controlled content; Thunderbird mail reading disables scripting.

CVSS severity8.8
High
EPSS probability1.18%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Use-after-free in Mozilla Firefox before 88, Firefox ESR before 78.10, and Thunderbird before 78.10 allows attackers to execute code. Responsive Design Mode retains and later dereferences references to objects after their storage has been freed, causing memory corruption. Exploitation requires a user to enable Responsive Design Mode and interact with attacker-controlled content; Thunderbird mail reading disables scripting.

The record

CVE
CVE-2021-23995
Published
Jun 24, 2021
Updated
Aug 3, 2024
Vendor
Mozilla
Product
Firefox
Classifications
CWE-672, T1203
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jun 24, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=unspecified <88

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mozilla · Firefoxversion=unspecified <88
Mozilla · Thunderbirdversion=unspecified <78.10
Mozilla · Firefox ESRversion=unspecified <78.10

Published CVSS scores

5.1NIST NVDAV:N/AC:H/Au:N/C:P/I:P/A:P
8.8NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-672Operation on a Resource after Expiration or Release
T1203Exploitation for Client Execution

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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